Method for producing a vessel from a preform, with feedback depending on the development point of the preform

ABSTRACT

Method for producing a vessel ( 1 ) by stretch blow-moulding in a mould ( 11 ) from a preform ( 3 ) made of plastic material, comprising the operations consisting in
     heating the preform ( 3 ) to a predetermined heating temperature (T);   introducing the preform ( 3 ) in the mould ( 11 );   stretching the preform ( 3 ) by means of a rod ( 16 ) displaced at a predetermined stretching speed (V E );   at a predetermined moment, called pre-blowing cue (t P ), controlling the opening of an electrovalve ( 22 ) for establishing a communication between the inside of the preform ( 3 ) and a source ( 20 ) of gas at a predetermined pre-blowing pressure (P P ) and a pre-blowing flow rate (D P );   measuring the pressure (P) inside the preform ( 3 );   detecting a pressure peak;   memorising the moment (t B ) at which this pressure peak occurs as well as the corresponding pressure (P B ) inside the preform ( 3 );   comparing the moment (t B ) and the pressure (P B ) of the peak detected with, respectively, a predetermined moment and pressure for a theoretical pressure peak;   if the measured peak and the theoretical peak are not confounded, modify at least one of the following parameters: pre-blowing pressure (P P ), pre-blowing flow rate (D P ), pre-blowing cue (t P ), stretching speed (V E ), heating temperature (T).

The invention relates to producing vessels by blow-moulding or stretch blow-moulding, from preforms made of plastic material.

In order to manufacture a vessel according to the blowing technique, we begin by heating a preform (whether entailing a preform or an intermediary vessel that has undergone a first blowing operation from a preform) at a temperature greater than the glass transition temperature of the material of the preform. The preform is then introduced into a mould, then the blowing of the preform is carried out by injecting therein a gas (such as air) under high pressure (generally greater than 30 bars). The stretch blow-moulding technique consists, other than the blowing, in stretching the preform using a sliding rod, in order in particular to minimise the offset of the vessel and to render uniform as much as possible the distribution of the material.

In order to prevent the preform from bursting during the blowing operation, prior to the blowing, a pre-blowing operation is generally provided, during which a gas under reduced pressure (generally between 5 and 16 bars) is injected into the preform. It is during the pre-blowing operation that is carried out the stretching of the preform, with the blowing intervening only afterwards.

In a context of industrial production, where manufacturing rates reach several tens of thousands of vessels per hour and per machine (a machine generally comprises several moulds mounted on a rotating carrousel), the duration of the method for producing a vessel, between the moment the preform is introduced into the mould and the moment the formed vessel is ejected, is of a few seconds. The duration of the pre-blowing is only of a few tenths of a second.

The constant desire of industrialists to reduce the quantity of material used for each vessel, combined with the velocity of the method, makes the manufacture delicate and can lead to substantial scrap rates. Among the main defects observed on the vessels is a poor distribution of the material. The difficulty that exists in combining saving material and the increase in the rates has led the industrialists to become more interested in the unfolding of the method by monitoring the variations in certain parameters over time, the main one being the pressure inside the preform, combined with the stretching speed.

Certain entities, such as Du Pont De Nemours in its American patent U.S. Pat. No. 4,042,657, suggest that there is a correlation between the pressure variations in the preform and the final form of the vessel, and recommend rejecting the vessels of which the pressure curve does not comply with a standard curve. If the proposal to reject the non-compliant vessels makes it possible to improve the overall quality of production, it puts a strain however on productivity and, in the hypothesis where the rejected vessels would not be recycled, leads to a wasting of material although it is desired to save it.

The invention in particular aims to overcome these disadvantages, by proposing a method making it possible to improve the quality of the vessels produced, while still limiting scrapping, and by maintaining—and even by increasing—the production rates.

To this effect, the invention proposes, according to a first object, a method for producing a vessel by stretch-blow moulding in a mould from a preform made of plastic material, comprising the following steps:

-   -   heating the preform to a predetermined heating temperature;     -   introducing the preform into the mould;     -   stretching the preform using a rod displaced at a predetermined         stretching speed;     -   at a predetermined moment, called pre-blowing cue, controlling         the opening of an electrovalve for establishing a communication         between the inside of the preform and a source of gas at a         predetermined pre-blowing pressure and pre-blowing flow rate;     -   measuring the pressure inside the preform;     -   detecting a pressure peak;     -   memorising the moment at which this pressure peak occurs as well         as the corresponding pressure inside the preform;     -   comparing the moment and the pressure peak detected with,         respectively, a predetermined moment and pressure for a         theoretical pressure peak;     -   if the measured peak and the theoretical peak are not         confounded, modify at least one of the following parameters:         pre-blowing pressure, pre-blowing flow rate, pre-blowing cue,         stretching speed, heating temperature.

It has been observed that not only the vessels as such produced have, in relation to the vessels obtained by the conventional methods, a quality that on the average is higher thanks to a better distribution of the material, but also that this quality is more constant over time.

The following adjusting operations can be carried out, according to the situations observed:

-   -   if the moment and the pressure of the peak detected are,         respectively, before and less than the theoretical moment and         pressure of the peak, decrease the heating temperature.     -   if the moment and the pressure of the peak detected are,         respectively, before and equal to the theoretical moment and         pressure of the peak, decrease the pressure or the pre-blowing         flow rate. An additional operation can in this case consist in         decreasing the heating temperature.     -   if the moment and the pressure of the peak detected are,         respectively, before and greater than the theoretical moment and         pressure of the peak, decrease the pressure or the pre-blowing         flow rate.     -   if the moment and the pressure of the peak detected are,         respectively, equal and greater than the theoretical moment and         pressure of the peak, decrease the pressure or the pre-blowing         flow rate. In this case, an additional operation can consist in         increasing the heating temperature.     -   if the moment and the pressure of the peak detected are,         respectively, after and greater than the theoretical moment and         pressure of the peak, increase the heating temperature. In this         case, an additional operation can consist in decreasing the         pre-blowing flow rate and/or in advancing the pre-blowing cue.     -   if the moment and the pressure of the peak detected are,         respectively, after and equal to the theoretical moment and         pressure of the peak, increase the pressure or the pre-blowing         flow rate. In this case, an additional operation can consist in         increasing the heating temperature.     -   if the moment and the pressure of the peak detected are,         respectively, after and less than the theoretical moment and         pressure of the peak, increase the pressure or the pre-blowing         flow rate.     -   if the moment and the pressure of the peak detected are,         respectively, equal and less than the theoretical moment and         pressure of the peak, increase the pressure or the pre-blowing         flow rate. In this case, an additional operation can consist in         decreasing the heating temperature.

The invention further proposes a method for producing a vessel by stretch-blow moulding in a mould from a preform made of plastic material, comprising the following steps:

-   -   heating the preform to a predetermined heating temperature;     -   introducing the preform into the mould;     -   stretching the preform using a rod displaced at a predetermined         stretching speed;     -   at a predetermined moment, called pre-blowing cue, controlling         the opening of an electrovalve for establishing a communication         between the inside of the preform and a source of gas at a         predetermined pre-blowing pressure and pre-blowing flow rate;     -   measuring the pressure inside the preform;     -   detecting a pressure peak;     -   memorising the moment at which this pressure peak occurs;     -   comparing the moment of the peak detected with a predetermined         moment of a theoretical pressure peak;     -   if the measured moment and the theoretical moment are not         confounded, modify at least one of the following parameters:         pre-blowing pressure (P_(P)), pre-blowing flow rate, pre-blowing         cue, stretching speed, heating temperature.

The invention proposes moreover a method for producing a vessel by stretch-blow moulding in a mould from a preform made of plastic material, comprising the following steps:

-   -   heating the preform to a predetermined heating temperature;     -   introducing the preform into the mould;     -   stretching the preform using a rod displaced at a predetermined         stretching speed;     -   at a predetermined moment, called pre-blowing cue, controlling         the opening of an electrovalve for establishing a communication         between the inside of the preform and a source of gas at a         predetermined pre-blowing pressure and pre-blowing flow rate;     -   measuring the pressure inside the preform;     -   detecting a pressure peak;     -   memorising the pressure inside the preform corresponding to the         pressure peak;     -   comparing the pressure of the peak detected with a predetermined         pressure of a theoretical pressure peak;     -   if the pressure measured and the theoretical pressure are not         confounded, modify at least one of the following parameters:         pre-blowing pressure, pre-blowing flow rate, pre-blowing cue,         stretching speed, heating temperature.

An additional purpose of the invention is a machine for manufacturing vessels from preforms made of a plastic material, comprising:

-   -   an oven provided with means of heating preforms;     -   a mould having a cavity intended to receive a preform;     -   a source of gas at a predetermined pre-blowing pressure;     -   an electrovalve able to establish a communication between the         inside of the preform, received in the cavity, and said source         of gas, at a predetermined pre-blowing flow rate;     -   means for controlling the opening of the electrovalve at a         predetermined moment called pre-blowing cue;     -   a sensor able to measure the pressure inside the preform;     -   means for detecting the so-called development moment, when a         pressure peak occurs in the preform;     -   means for comparing the development moment and the so-called         development pressure, inside the preform at this moment, with,         respectively, a theoretical development moment and pressure;     -   means for adjusting the pre-blowing flow rate according to the         result of this comparison.

According to an embodiment, this machine further comprises:

-   -   means for adjusting the pre-blowing pressure according to the         comparison between the development moment and pressure with the         theoretical development moment and pressure;     -   means for adjusting the heating temperature according to the         comparison between the development instant and pressure with the         theoretical development moment and pressure.     -   means for adjusting the pre-blowing cue according to the         comparison between the development instant and pressure with the         theoretical development moment and pressure.

Furthermore, the machine comprises more preferably:

-   -   a stretching rod that can be displaced at a predetermined         stretching speed;     -   means for adjusting the stretching speed according to the         comparison between the development instant and pressure with the         theoretical development moment and pressure.

An additional purpose of the invention is a computer programme product intended to be implemented on a machine for manufacturing vessels such as described hereinabove, comprising instructions for:

-   -   controlling the opening of the electrovalve;     -   taking into account the development moment;     -   taking into account the development pressure;     -   taking into account the theoretical moment of development;     -   taking into account the theoretical development pressure;     -   comparing the development moment with the theoretical         development moment;     -   comparing the development pressure with the theoretical         development pressure;     -   adjusting at least one of the following parameters: pre-blowing         pressure, pre-blowing flow rate, pre-blowing cue, stretching         speed, heating temperature according to the result of these         comparisons.

Other objects and advantages of the invention shall appear when reading the description provided hereinafter in reference to the annexed drawings wherein:

FIG. 1 is a schematic view showing a machine for manufacturing vessels;

FIG. 2 is a schematic cross-section view showing partially a moulding unit in the machine in FIG. 1;

FIG. 3 is a diagram whereon is plotted a curve showing the variations in the pressure inside a preform during the manufacture of a vessel;

FIG. 4 is a diagram whereon is plotted a curve showing the variations in the pressure inside a preform during the pre-blowing operation;

FIGS. 5A to 5H are cross-section view showing the preform in the mould, at different stages during the pre-blowing;

FIG. 6 is a diagram showing the superposition of two pressure curves corresponding respectively to two different moments of controlling the opening of a pre-blowing electrovalve;

FIG. 7 is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the pressure or of the pre-blowing flow rate;

FIG. 8 is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the stretching speed;

FIG. 9 is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the heating temperature of the preform; and

FIG. 10 is a diagram showing the superposition of two pressure curves corresponding respectively to two moments of controlling the opening of a blowing electrovalve.

FIG. 1 shows a machine 1 for the manufacturing of vessels 2 from preforms 3 made of plastic material such as PET (polyethylene terephtalate). According to a preferred embodiment, the preforms 3 are preforms from which the final vessels 2 are directly obtained, without passing by an intermediary vessel. Each preform 3 comprises a neck 4 (which does not undergo any deformation during the formation of the vessel 2), and a cylindrical body 5 terminated by a hemispheric base 6.

This machine 1 comprises an oven 7 of the tunnel type, wherein the preforms 3 are continuously heated to a temperature greater than the glass transition temperature of their material. In the case of PET, of which the glass transition temperature is in the neighbourhood of 80° C., the heating temperature is more preferably between 100° C. and 140° C.

The machine 1 further comprises a plurality of moulding units 8 mounted on a rotating carrousel 9 arranged at the outlet of the oven 7 with interposition of a transfer wheel 10 providing the synchronisation of the heating and moulding operations of the preforms 3.

Each moulding unit 8 comprises a mould 11, made of steel or aluminium alloy, comprising two half-moulds 12, 13 and a mould base 14 which together define an internal cavity 15, intended to receive a preform 3 coming from the oven 7.

Each moulding unit 8 further comprises:

-   -   a stretch rod 16 mounted sliding in relation to mould 11 along a         main X axis (generally of revolution) of the latter, between a         high position (FIG. 2) making it possible to introduce the         preform 3 and a low position (FIG. 5H) where, at the end of the         stretching of the preform 3, the rod 16 reaches the mould base         14 pushing therein the base 6 of the latter,     -   a case 17 defining a nozzle 18 wherein slides the rod 16 and         which, during the manufacture of the vessel 2, interacts with         the neck 4 of the preform 3.

The moulding unit 8 further comprises several fluid circuits exiting into the nozzle 18 via the case 17, i.e.:

-   -   a medium pressure pre-blowing air circuit 19 (between 5 and 16         bars), this circuit 19 comprising a source 20 of pre-blowing air         and a duct 21 (which can be formed at least partially in the         case 17) connecting this source 20 to the nozzle 18 with         interposition of a first so-called pre-blowing electrovalve 22,     -   a high pressure blowing air circuit 23 (between 30 and 40 bars),         comprising a source 24 of blowing air and a duct 25 (which can         be formed at least partially in the case 17) connecting this         source 24 to the nozzle 18 with interposition of a second         so-called blowing electrovalve 26,     -   a degassing circuit 27 comprising an outlet for venting 28 and a         duct 29 connecting the nozzle 18 to this outlet 28 with         interposition of a third so-called degassing electrovalve 30

The electrovalves 22, 26, 30 are connected electrically to a control unit 31 which controls the opening and the closing of it (duly taking their response times into account). These electrovalves 22, 26, 30 can be arranged at a distance from the case 17; however, for more compactness, they are more preferably at least partially included in the latter, in accordance with the embodiment described in the French patent application published under number FR 2 872 082 or in the equivalent international application published under the number WO 2006/008380, both in the name of the applicant and to which those skilled in the art may refer.

The moulding unit 8 is moreover provided with a pressure sensor 32 with which is measured the pressure inside the preform 3 during the manufacture of the vessel 2. To this effect, the sensor 32, connected to the control unit 31 that memorises the pressure measurements taken, is provided with a substantial portion 33 exiting into the nozzle 18 (where the pressure is identical to that inside the preform 3).

The manufacture of a vessel 2 from a preform 3 is carried out as follows.

The preform 3, mounted on a conveyor, is first of all introduced in the oven 7 where it is heated, neck 4 downwards, in the conditions described hereinabove. At the outlet of the oven 7, the preform 3 is grasped by a clamp of the transfer wheel 10 and introduced, after having been turned over to have the neck 4 directed upwards, into a mould 11 opened beforehand. In this position, the stretch rod 16 is in high position.

As the carrousel 9 rotates, the mould 11 closes on the preform 3. Once the mould 11 is closed (FIG. 5A), begins, at a predetermined moment called starting cue, the descending of the stretch rod 16 into the moulding cavity 15 (FIG. 5B). The starting cue constitutes the origin of the axis of time (abscissas) in FIGS. 3, 4 and 6 to 10. Starting from this moment the pressure P inside the preform is measured continuously, each value being memorised in the control unit 31. The term “continuously” means that the period during which the pressure measurements are taken is low in light of the time needed for the pressure variations in the preform 3. To this effect, a high-performance sensor 32 will be chosen, making it possible to take pressure measurements at a period of less than or equal to 5 ms (millisecond), more preferably less than or equal to 2 ms, with the ideal being to be able to take the measurements at a period of 1 ms.

To starting cue corresponds a predefined angular position of the mould 11 on the circumference of the carrousel 9. According to a first embodiment, the controlling of the displacement of the stretch rod 16 is purely mechanical, carried out using a cam against which rolls or slides a cam follower integral with the stretch rod 16. In this case, the moment when the stretch rod 16 begins its descent is confounded with the starting cue, which corresponds to the moment when the cam follower enters into contact with the cam.

According to a second embodiment, the displacement of the stretch rod 16 is provided by an electromechanical system controlled by the control unit 31. In this case, the moment when the descent of the stretch rod 16 begins is shifted in relation to starting cue by an offset value equal to the response time of the electromechanical system. This offset value, provided by the manufacturer of the system, is taken into account in the adjustment of the starting cue so that the descent of the stretch rod 16 starts as soon as possible after the closing of the mould 11. V_(E) denotes the displacement speed of the stretch rod 16; this speed is also called stretching speed.

At a predetermined moment, called pre-blowing cue t_(P), the control unit 31 controls the opening of the pre-blowing electrovalve 22 for establishing a communication between the inside of the preform 3 and the source 20 of pre-blowing air.

The moment, called real moment of the beginning of the pre-blowing and noted as t_(A), from which the pressure P in the preform 3 begins to increase, registers a delay in relation to the pre-blowing cue t_(P), equal to the response time of the pre-blowing electrovalve 22.

This delay, either provided by the manufacturer of the electrovalve 22, or measured, is taken into account in the adjustment of the pre-blowing cue t_(P), so that the real moment t_(A) of the beginning of pre-blowing begins after the moment when the stretch rod 16 reaches the base of the preform 3, with this moment able to be calculated simply from the descending speed (known) of the stretch rod 16 and from the length (also known) of the preform 3.

In FIGS. 4 and 6, the point of the curve having for abscissa the real moment t_(A) of the beginning of the pre-blowing is noted as A. To this point corresponds a pressure (relative) substantially nil, the pressure inside the preform 3 during its introduction into the mould 11 being indeed substantially equal to the atmospheric pressure.

From point A begins an axial expansion phase of the preform 3. As can be seen in FIGS. 4 and 6, the slope of the pressure curve at point A is nil on the left, and positive on the right. From point A, the pressure in the preform 3 increase as it is stretched, the flow of air introduced in the preform 3 being higher than the increasing of the internal volume of the preform 3 (which undergoes no radial expansion, Cf. FIG. 5C), up to a moment, called development moment and noted as t_(B), when the plastic flow threshold of the preform 3 has been reached. At this moment t_(B) a pressure peak occurs, the corresponding point, called development point, being noted as B in FIGS. 4 and 7 to 9. At point B the slope of the blowing curve is nil, this slope reversing from left (where it is positive) to right (where it is negative) of point B. The corresponding pressure, called development pressure, is noted as P_(B).

Starting from the development moment t_(B) begins an axial and radial expansion phase of the preform. At the beginning of this phase, immediately after the development point, a momentary drop ΔP in the pressure P is noted inside the preform (FIG. 5D). This drop ΔP in pressure is due to the sudden beginning of the radial expansion of the preform 3, which, with its plastic flow threshold crossed, thus undergoes a plastic deformation.

After having dropped by a value ΔP of approximately 1 bar, the pressure P is maintained, with the increase in the volume of the preform 3 offsetting the flow of air injected in the latter (FIGS. 5E, 5F). This radial and axial expansion phase ends when the stretching ends, the stretch rod 16 having reached its low position where it pushes the base 6 of the preform 3 against the mould base 14 (FIG. 5G).

The combination of a radial deformation and of an axial deformation of the preform 3 leads to a bi-orientation (axial and radial) of the macromolecules of the material, which in particular results in increasing the mechanical strength of the vessel 2 and in decreasing its permeability to gases and liquids. For further information, refer to the work of Rosato and DiMattia, “Blow Molding Handbook” (2^(nd) edition, Hanser, 2004).

After the end of the stretching, the rod 16 being maintained in its low position, takes place a radial expansion phase of the preform 3 during which no inflection of the pressure P is noted, which continues to be stable. That is why at the moment, noted as t_(E), when the stretching ends, no significant point corresponds on the pressure curve, this moment t_(E) being however known as it is predetermined by the value (which may be adjustable) of the stretching speed V_(E) and the height of the vessel 2. This radial expansion phase ends at a moment, called moment of expansion end, noted as t_(C), when the preform 3 has reached the surface of the cavity 15 over substantially all of its height, in other words when the internal volume of the preform 3 is almost equal to the final volume of the vessel 2, if localised zones are accepted where the contact between the preform 3 and the cavity 15 is not complete, for example in zones wherein are arranged ribs, grooves or other recesses aiming to confer upon the vessel 2 better strength or a certain aesthetics (FIG. 5H). To the moment t_(C) of expansion end corresponds on the pressure curve a significant point, called expansion end point and noted as C in FIGS. 4 and 10, where the curve sags and has an increase in its slope. The pressure at the point C, called expansion end pressure, is noted as P_(C).

From the expansion end point C begins a growth phase in the pressure P in the preform 3, at a constant volume. The rate, noted as D_(P) and the pressure, noted as P_(P), of pre-blowing air being maintained constant, the pressure P in the preform 3 increases in a linear manner up to a moment, noted as t_(D), of the beginning of blowing from which starts a blowing phase: communication of the inside of the preform 3 is established with the source 24 of blowing air, the pressure P in the preform 3 increasing suddenly up to the value of the blowing pressure. In light of the response time of the blowing electrovalve 26, the moment t_(D), of the beginning of blowing registers a slight delay in relation to a so-called blowing cue and noted as t_(S), to which the control unit 31 controls simultaneously the closing of the pre-blowing electrovalve 22 and the opening of the blowing electrovalve 26. To the moment t_(D) of beginning of blowing corresponds on the pressure curve a significant point, called blowing starting point and noted as D, when the curve sags and has a substantial increase in its slope.

The inventors feel that the pre-blowing is a determining factor in the quality of the final vessel. Controlling pre-blowing is carried out by the intermediary of the plot of the corresponding pressure curve. Ideally, all the vessels are identical. In reality, the repeatability of the method for producing is not provided due to variations in physical parameters (pressure, temperature) and mechanical parameters (electrovalve response time, obstruction of the pneumatic circuits, leaks in the pre-blowing and blowing circuits, etc.) which can have an impact on the different moulding units 8 of the machine 1 during the production of vessels 2. That is why substantial differences are noted between the vessels 2. In order to provide the constancy of the quality of the vessels obtained, in order to place them in conformity as much as is possible with a pre-established standard, the inventors propose to provide a continuous adjustment of the pre-blowing operations through feedback on at least one of the following parameters:

-   -   heating temperature T of the preforms;     -   pre-blowing pressure P_(P);     -   pre-blowing rate D_(P);     -   stretching speed V_(E);     -   pre-blowing cue t_(P);     -   blowing cue t_(S).

Through hypothesis, we have a theoretical pre-blowing curve, (i.e. the portion of the pressure curve between the starting cue, taken as the origin of time, and the point D) for which the quality of the vessel obtained appears to be compliant with pre-established criteria including for example the mass of the base of the vessel, assumed to lie within a range of values, the uniformity of the thickness of the wall, the coaxiality of the base and of the neck, etc.

The inventors have considered that it is illusory to attempt to obtain for each vessel produces a curve that is strictly identical to the theoretical pre-blowing curve and consequently propose a simpler method for controlling while still remaining effective. According to this method, at least one characteristic point is selected on the theoretical curve, defined by a time/pressure ordered pair, deemed characteristic of the proper unfolding of the pre-blowing operations, and a tolerance zone is defined around this point, the combination of a time tolerance (along the x axis) and of a pressure tolerance (along the y axis), in order to take into account in particular the error margins on the measurements taken.

For the or each characteristic point, the real point, such as results from the measurements taken during the production, is compared with the corresponding theoretical characteristic point. In practice, it is verified that the real point, such as is measured, is located within the tolerance zone defined around the theoretical characteristic point by checking that the moment of the real point is located in the time tolerance and that its pressure is located within the pressure tolerance. In the hypothesis where the real point is effectively located in the tolerance zone, the real and theoretical points are declared confounded. On the contrary, when the real point is located outside of the tolerance zone, the real and theoretical points are declared distinct, in which case, as we shall see hereinafter, feedback measurements can be taken in order to modify at least one of the parameters listed hereinabove in order to cause to converge, on the pressure curve of the following vessel, the real point towards the theoretical point.

In practice, points A, B, C and D are controlled (and are possibly acted upon), alone or in combination. We shall now describe how to control and act on each of these points.

Point A

Recall that the point A corresponds to the moment t_(A) from which the pressure in the preform 3 begins to increase, after the control unit 31 has controlled the opening of the pre-blowing electrovalve 22 (at the pre-blowing cue t_(P)) in order to establish communication between the inside of the preform 3 and the source 20 of pre-blowing air.

It is important that the position of the point A be substantially identical for all of the moulding units 8. The occurrence of the point A is therefore detected, i.e. the moment t_(A) is detected where the pressure P in the preform 3 begins to increase. (This measurement can be taken either during manufacture of the vessel 2, or a posteriori, directly on the pre-blowing curve.)

The real point A is then compared with the theoretical point A. Where the pressure at the point A is nil, in practice the moment t_(A) of the real point A is compared with the moment of the theoretical point A, i.e. the theoretical pre-blowing beginning moment, in order to check if the moment t_(A) of the real point A is located within the tolerances defined for the point A.

In the hypothesis where the real moment t_(A) of the beginning of pre-blowing follows (taking the tolerance into account) the theoretical pre-blowing beginning moment, the pre-blowing cue t_(P) is advanced.

In the hypothesis where on the contrary the real moment t_(A) of the beginning of pre-blowing precedes (taking the tolerance into account) the theoretical moment of the beginning of the pre-blowing, the pre-blowing cue t_(P) is delayed.

The two curves plotted in solid lines in FIG. 6 show two different settings of the pre-blowing cue t_(P). The left curve shows an advancing of the pre-blowing cue t_(P); the right curve shows on the contrary a delaying of the pre-blowing cue t_(P). The theoretical curve is, between the latter, plotted in dotted lines.

More precisely, it is preferable to measure the difference separating the real moment t_(A) of the beginning of pre-blowing of the theoretical pre-blowing beginning moment. And, if the real moment t_(A) of the beginning of pre-blowing follows—respectively precedes—the theoretical pre-blowing beginning moment, the pre-blowing cue t_(P) is advanced—respectively delayed—by the value of the difference.

An example with numbers is provided in the table hereinafter for the purposes of illustrating the above. The measurements shown correspond to the pressure curve in FIG. 4.

Value Parameter Type of vessel Bottle 1.5 l Material PET Heating temperature 120-140° C. Pre-blowing pressure 10 bars Stretching speed 2000 mm/s Pre-blowing cue 0.03 s Response time of the pre-blowing 0.01 s electrovalve Theoretical pre-blowing beginning ±0.005 s moment tolerance Measurement Theoretical pre-blowing beginning 0.04 s moment

In order to allow for the adjustment of the point A, the machine 1 comprises:

-   -   means for detecting the real moment t_(A) of the beginning of         pre-blowing;     -   means for comparing this moment t_(A) with the theoretical         pre-blowing beginning moment; and     -   means for adjusting the pre-blowing cue t_(P) according to the         result of this comparison.

The machine 1 may also comprise means for memorising a plurality of points A (for example approximately ten) detected on a series of manufactured vessels 2, in particular for the purposes of statistical study.

These means can have the form of instructions of a computer programme product implemented in the control unit 31 of the machine 1.

Point B

Recall that the point B corresponds to the plastic flow threshold of the preform 3, from which starts its radial expansion.

Adjusting the development moment t_(B) (abscissa of the point B) makes it possible to control the radial expansion of the preform 3. A premature occurrence, or on the contrary a late one, of the development point B can in the same way indicate a poor distribution of the material, with as a consequence a defect in the strength of the vessel 2.

The occurrence of the point B is therefore detected, i.e. the moment t_(B) when a pressure peak occurs in the preform 3 after the pre-blowing cue t_(P) (but before the blowing cue t_(S)) is detected. As with the point A, this measurement can be taken either during manufacture of the vessel 2, or more preferably a posteriori, directly on the pre-blowing curve.

The real point B is then compared with the theoretical point B. In practice, the real development moment t_(B) is compared with the theoretical moment of development on the one hand and, on the other hand, the real development pressure P_(B) is compared with the theoretical development pressure in order to check that both lie within the tolerances defined for the point B.

The real point B is declared confounded with the theoretical point B when it is located within the tolerance zone defined around the theoretical point B.

When on the contrary the real point B is not confounded with the theoretical point B, i.e. it is located outside of the tolerance zone, by feedback a modification is carried out on at least one of the following parameters:

-   -   pre-blowing cue t_(P),     -   pre-blowing pressure P_(P),     -   pre-blowing flow rate D_(P),     -   stretching speed V_(E),     -   heating temperature T of the preforms.

Several cases can occur, according to whether the real development moment t_(B) precedes, is confounded (i.e. is within the tolerance zone) or follows the theoretical development moment, and according to whether the real development pressure P_(B) is less than, equal (i.e. within the tolerance zone) or greater than the theoretical development pressure.

We have seen that modifying the pre-blowing cue t_(P) produces a shift in the point A, towards the right on the axis of time when the pre-blowing cue t_(P) is delayed, or on the contrary towards the left when the pre-blowing cue t_(P) is advanced. In fact, this modification has an impact on the entire pre-blowing curve, which is then shifted in the same direction as the point A. The modification of the pre-blowing cue t_(P) thus impacts the development moment t_(B), i.e. the abscissa of the point B. In the hypothesis where the real development moment t_(B) is, either before or after the theoretical development moment, a simple measurement could consist in delaying or, respectively, advancing the pre-blowing cue t_(P). However, the real moment t_(A) of the beginning of pre-blowing (abscissa of the point A) would then be affected needlessly. That is why it appears preferable that feedback on the pre-blowing cue t_(P) in order to modify the development moment t_(B) be carried out only when the modification of the other parameters is not enough to return the development point B to the tolerance zone.

The influence of the other parameters is shown in FIGS. 7, 8 and 9.

FIG. 7 shows the influence of the pressure P_(P) or of the rate D_(P) of pre-blowing on the unfolding of the pre-blowing. It has been observed that the influence of the pressure P_(P) is similar to that of the rate D_(P). The joint influence of these two parameters can therefore be expressed as the influence of their product P_(P)×D_(P): the increase in this product can result from an increase in the pre-blowing pressure P_(P) at a constant rate D_(P), from an increase in the pre-blowing rate D_(P) at a constant pressure P_(P), or from a simultaneous increase in the both D_(P) and P_(P); on the contrary, a decrease in the product P_(P)×D_(P) can result from a decrease in the pre-blowing pressure P_(P) at a constant rate D_(P), a decrease in the pre-blowing rate D_(P) at a constant pressure P_(P), or a simultaneous decrease in the pressure D_(P) and in the rate D_(P), being understood that increasing one while decreasing the other does not appear to be useful.

In FIG. 7, the left curve corresponds to a value of the product P_(P)×D_(P) greater than that corresponding to the right curve: it is observed that an increase in the product P_(P)×D_(P) results in an increase in the slope of the pressure curve at the point A (i.e. at the real moment t_(A) of the beginning of pre-blowing), an advancing of the development moment t_(B) and an increase in the development pressure P_(B) (on the curve, the point B is therefore displaced towards the top and towards the left). On the contrary, a decrease in the product P_(P)×D_(P) results in a decrease in the slope of the pressure curve at the point A, a delaying of the development moment t_(B) and a decrease in the development pressure P_(B) (on the curve, the point B is therefore displaced towards the bottom and towards the right).

FIG. 8 shows the influence of the stretching speed V_(E) on the unfolding of the pre-blowing. To the lower curve corresponds a stretching speed V_(E) greater than that corresponding to the upper curve. It is observed that the increase in the stretching speed V_(E) results in an advancing of the development moment t_(B) and a decrease in the development pressure P_(B) (on the curve, the point B is therefore displaced towards the bottom and towards the left), while the decrease in the stretching speed V_(E) results in a delaying of the development moment t_(B) and an increase in the development pressure P_(B) (on the curve, the point B is therefore displaced towards the top and towards the right). No influence of the stretching speed V_(E) on the slope of the pressure curve between the point A and the point B is however noted.

FIG. 9 shows the influence of the heating temperature T on the course of the pre-blowing. To the lower curve corresponds a heating temperature T greater than that corresponding to the upper curve. It is observed that the increase in the heating temperature T has an impact that is similar to the increase in the stretching speed V_(E): advancing of the development moment t_(B) and a decrease in the development pressure P_(B), while the decrease in the heating temperature T has an influence similar to the decrease of the stretching speed V_(E): delaying of the development moment t_(B) and an increase in the development pressure P_(B).

We shall present hereinafter the recommended measures to return, in each possible case, the development point B to its tolerance zone, according to the observed influence of the parameters listed hereinabove and of the priority given to certain parameters over others. In practice, priority is given to the pressure P_(P) or to the rate D_(P) over the heating temperature T. In light of its influence on the entire curve, and in particular on the real point A of the beginning of pre-blowing, the pre-blowing cue t_(P) is, in practice, modified only when modifications of the pressure P_(P), in the rate D_(P) or in the heating temperature T have not been enough to return the development point B to its tolerance zone. As for the stretching speed V_(E), this is modified only in order to refine the positioning of the development point B (and with the condition that the machine 1 be provided with means of adjusting the stretching speed V_(E)).

Development Development Modification of the moment pressure parameters Before Lower Decrease T In the tolerance Decrease D_(P) or P_(P) zone Decrease T Higher Decrease D_(P) or P_(P) In the Lower Decrease T tolerance Increase D_(P) or P_(P) zone In the tolerance zone Higher Increase T Decrease D_(P) or P_(P) After Lower Increase D_(P) or P_(P) In the tolerance Increase D_(P) or P_(P) zone Increase T Higher Increase T Where applicable: Decrease D_(P) or P_(P), Advance pre-blowing cue

An example with numbers is provided in the table hereinafter, in order to show the values of the moment and of the development pressure. The measurements shown correspond to the pressure curve in FIG. 4.

Value Parameter Type of vessel Bottle 1.5 l Material PET Heating temperature 120-140° C. Pre-blowing pressure 10 bars Pre-blowing flow rate* 40 l/s Stretching speed 2000 mm/s Pre-blowing cue 0.03 s Development moment tolerance ±0.005 s Measurement Development pressure tolerance 0.5 bar Development moment (point B) 0.07 s Development pressure (point B) 5 bars *given in normolitres per second (equivalent of the litre per second at atmospheric pressure)

Note that the displacement of the development point B (especially its abscissa, the development moment t_(B)) has an incidence on the moment t_(C) of expansion end (abscissa of the point C). The modifications of parameters that affect the position of the point B can thus result in having to modify the position of the point D (i.e., according to the case, in advancing or in delaying the blowing cue t_(S)) which can indeed be located shifted in relation to point C; the corresponding adjustments are presented hereinafter.

In order to allow for the adjustment of the point B, the machine 1 comprises:

-   -   means for detecting the development moment t_(B);     -   means for comparing the development moment t_(B) and the         development pressure P_(B) with, respectively, the theoretical         development moment and pressure;     -   means for adjusting the pre-blowing rate D_(P) according to this         comparison.

These means can take the form of instructions of a computer programme product implemented in the control unit 31 of the machine 1.

Furthermore, in order to effectively proceed in adjusting of the pre-blowing rate D_(P), the machine 1 comprises for example a flow regulator 34 mounted on the pre-blowing electrovalve and controlled by the control unit 31 (cf. FIG. 2).

The machine 1 can, according to the parameters that are chosen to adjust, include, for example in the form of additional instructions from the computer programme:

-   -   means for adjusting the pre-blowing pressure P_(P) according to         the comparison between the real point B and the theoretical         point B;     -   means for adjusting the heating temperature T according to the         comparison between the real point B and the theoretical point B;     -   means for adjusting the pre-blowing cue t_(P) according to the         comparison between the real point B and the theoretical point B;     -   means for adjusting the stretching speed V_(E) according to the         comparison between the real point B and the theoretical point B.

Alternatively, it can be desired to adjust the development moment t_(B) independently of the development pressure P_(B), or reciprocally adjust the development pressure P_(B) independently of the development moment t_(B).

In the first case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure P_(P), pre-blowing flow rate D_(P), pre-blowing cue t_(P), stretching speed V_(E), heating temperature T in order to advance or, on the contrary, delay the development moment t_(B).

In the second case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure P_(P), pre-blowing flow rate D_(P), pre-blowing cue t_(P), stretching speed V_(E), heating temperature T in order to increase or, on the contrary, decrease the development pressure P_(B).

The machine 1 may also comprise means for memorising a plurality of points B (for example approximately ten) detected on a series of manufactured vessels 2, in particular for the purposes of statistical study.

Points C, D

Recall that the point C corresponds to the end of the expansion of the preform 3, when the latter has reached the surface of the cavity 15 over substantially all of its height, i.e. when the internal volume of the preform 3 has almost reached the final volume of the vessel 2. At the point C, the pressure curve sags and has an increase in its slope.

Once the expansion of the preform 3 is terminated, it is useless to continue to inject pre-blowing air therein, of which the pressure P_(P) is not enough to allow the material (which continues to cool and is therefore less and less deformable) to perfectly hug the surface of the cavity 15: such an operation therefore constitutes a loss of time that puts a strain on the productivity with no incidence on the quality of the vessel 2. It therefore appears desirable to start the blowing as soon as the expansion of the preform 3 has completed. In other words, it is desirable to take measurements in order to converge, on the pressure curve, the point D (beginning of the blowing) towards the point C (expansion end of the preform 3).

The occurrence of the point C is therefore detected, i.e. the moment t_(C) from which the pressure in the preform 3 begins to increase in a linear manner (moment of expansion end). Then, in the hypothesis where the blowing cue t_(P) follows the moment of expansion end, the blowing cue is advanced in order to converge the point D towards the point C.

More precisely, the delay separating the moment t_(C) of expansion end from the blowing cue t_(S) is measured, and the blowing cue t_(S) is advanced by a value equal to this delay. In reality, the response time of the blowing electrovalve 26 must be taken into account, due to which the real moment t_(D) of the beginning of blowing (abscissa of the point D) is slightly after the blowing cue t_(S).

In other words, either the delay separating the moment t_(C) of expansion end from the moment t_(D) of the beginning of blowing is measured and the blowing cue t_(S) is advanced by a value equal to this delay, or the delay separating the moment t_(C) of expansion end from the blowing cue t_(S) is measured and the blowing cue t_(S) is advanced by a value equal to this delay, increased by the response time of the blowing electrovalve 26.

As with points A and B, a certain tolerance is accepted for the point C. In other words, the points C and D are assumed to be confounded when the delay separating the moment t_(C) of expansion end from the moment t_(D) of the beginning of blowing (or the delay separating the moment t_(C) of expansion end from the blowing cue t_(S) increased by the response time of the blowing electrovalve 26) is less than a predetermined value.

In FIG. 10, two curves are shown: the one on the right corresponds to the presence of a difference between the moment t_(C) of the expansion end and the moment t_(D) of the beginning of blowing. The left curve, in dotted lines, corresponds to an adjustment carried out through feedback on the base of the right curve: the blowing cue t_(S) has been advanced by the value of the difference separating on the right curve the moment t_(C) from the moment t_(D).

An example with numbers is provided in the table hereinafter for the purposes of illustrating the above. The measurements shown correspond to the pressure curve in FIG. 4.

Value Parameter Type of vessel Bottle 1.5 l Material PET Heating temperature 120-140° C. Pre-blowing pressure 10 bars Stretching speed 2000 mm/s Pre-blowing cue 0.03 s Response time of the pre-blowing 0.01 s electrovalve Response time of the blowing 0.01 s electrovalve Point C tolerance ±0.005 s Measurement Moment of the beginning of pre-blowing 0.04 s (point A) Development moment (point B) 0.07 s Moment of stretching end 0.15 s Moment of expansion end (point C) 0.18 s

In order to allow for the adjustment of the point D, the machine 1 comprises:

-   -   means for detecting the moment t_(C) of the expansion end; and     -   means for adjusting the blowing cue t_(S) according to the         moment t_(C) of the expansion end.

More precisely, the machine 1 can include:

-   -   means for detecting the moment t_(D) of the beginning of         blowing; and     -   means for comparing the moment t_(C) of the expansion end with         the moment t_(D) of the beginning of blowing, the means for         adjusting being provided in order to advance the blowing cue         t_(S) by the value of the difference between them.

The machine 1 may also comprise means for memorising a plurality of points C and/or D (for example approximately ten) detected on a series of manufactured vessels 2, in particular for the purposes of statistical study.

Alternatively (or as a complement), the machine 1 can include means for comparing the moment t_(C) of the expansion end with the pre-blowing cue t_(S), the means for adjusting being provided in order to advance the blowing cue t_(S) by the value of the difference between them, increased by the response time of the blowing electrovalve 26.

These means can have the form of instructions of a computer programme product implemented in the control unit 31. 

1. Method for producing a vessel (2) by stretch blow-moulding in a mould (11) from a preform (3) made of plastic material, comprising the operations consisting in: heating the preform (3) to a predetermined heating temperature (T); introducing the preform (3) in the mould (11); stretching the preform (3) by means of a rod (16) displaced at a predetermined stretching speed (V_(E)); at a predetermined moment, called pre-blowing cue (t_(P)), controlling the opening of an electrovalve (22) for establishing a communication between the inside of the preform (3) and a source (20) of gas at a predetermined pre-blowing pressure (P_(P)) and a pre-blowing flow rate (D_(P)); measuring the pressure (P) inside the preform (3); detecting a pressure peak; memorising the moment (t_(B)) at which this pressure peak occurs as well as the corresponding pressure (P_(B)) inside the preform (3); comparing the moment (t_(B)) and the pressure (P_(B)) of the peak detected with, respectively, a predetermined moment and pressure for a theoretical pressure peak; if the measured peak and the theoretical peak are not confounded, modify at least one of the following parameters: pre-blowing pressure (P_(P)), pre-blowing flow rate (D_(P)), pre-blowing cue (t_(P)), stretching speed (V_(E)), heating temperature (T).
 2. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, before and less than the theoretical moment and pressure of the peak, an operation consisting in decreasing the heating temperature (T).
 3. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, before and equal to the theoretical moment and pressure of the peak, an operation consisting in decreasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 4. Method set forth in claim 3, which comprises an additional operation consisting in decreasing the heating temperature (T).
 5. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, before and greater than the theoretical moment and pressure of the peak, an operation consisting in decreasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 6. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, equal and greater than the theoretical moment and pressure of the peak, an operation consisting in decreasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 7. Method set forth in claim 6, which comprises an additional operation consisting in increasing the heating temperature (T).
 8. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, after and greater than the theoretical moment and pressure of the peak, an operation consisting in increasing the heating temperature (T).
 9. Method set forth in claim 8, which comprises an additional operation consisting in decreasing the pre-blowing flow rate (D_(P)).
 10. Method set forth in claim 8, which comprises an additional operation consisting in advancing the pre-blowing cue (t_(P)).
 11. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, after and equal to the theoretical moment and pressure of the peak, an operation consisting in increasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 12. Method set forth in claim 11, which comprises an additional operation consisting in increasing the heating temperature (T).
 13. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, after and less than the theoretical moment and pressure of the peak, an operation consisting in increasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 14. Method set forth in claim 1, which comprises, if the moment (t_(B)) and the pressure (P_(B)) of the peak detected are, respectively, equal and less than the theoretical moment and pressure of the peak, an operation consisting in increasing the pressure (P_(P)) or the pre-blowing flow rate (D_(P)).
 15. Method set forth in claim 14, which comprises an additional operation consisting in decreasing the heating temperature (T).
 16. Method for producing a vessel (2) by stretch blow-moulding in a mould (11) from a preform (3) made of plastic material, comprising the operations consisting in: heating the preform (3) to a predetermined heating temperature (T); introducing the preform (3) in the mould (11); stretching the preform (3) by means of a rod (16) displaced at a predetermined stretching speed (V_(E)); at a predetermined moment, called pre-blowing cue (t_(P)), controlling the opening of an electrovalve (22) for establishing a communication between the inside of the preform (3) and a source (20) of gas at a predetermined pre-blowing pressure (P_(P)) and a pre-blowing flow rate (D_(P)); measuring the pressure (P) inside the preform (3); detecting a pressure peak; memorising the moment (t_(B)) at which this pressure peak occurs; comparing the moment (t_(B)) of the peak detected with a predetermined moment for a theoretical pressure peak; if the measured moment (t_(B)) and the theoretical moment are not confounded, modify at least one of the following parameters: pre-blowing pressure (P_(P)), pre-blowing flow rate (D_(P)), pre-blowing cue (t_(P)), stretching speed (V_(E)), heating temperature (T).
 17. Method for producing a vessel (2) by stretch blow-moulding in a mould (11) from a preform (3) made of plastic material, comprising the operations consisting in: heating the preform (3) to a predetermined heating temperature (T); introducing the preform (3) in the mould (11); stretching the preform (3) by means of a rod (16) displaced at a predetermined stretching speed (V_(E)); at a predetermined moment, called pre-blowing cue (t_(P)), controlling the opening of an electrovalve (22) for establishing a communication between the inside of the preform (3) and a source (20) of gas at a predetermined pre-blowing pressure (P_(P)) and a pre-blowing flow rate (D_(P)); measuring the pressure (P) inside the preform (3); detecting a pressure peak; memorising the pressure (P_(B)) inside the preform corresponding to the pressure peak; comparing the pressure (P_(B)) of the peak detected with a predetermined pressure for a theoretical pressure peak; if the measured pressure (P_(B)) and the theoretical pressure are not confounded, modify at least one of the following parameters: pre-blowing pressure (P_(P)), pre-blowing flow rate (D_(P)), pre-blowing cue (t_(P)), stretching speed (V_(E)), heating temperature (T).
 18. Machine (1) for manufacturing vessels (2) from preforms (3) made of plastic material, comprising: an oven (7) provided with means of heating preforms (3); a mould (11) having a cavity (15) intended to receive a preform (3); a source (20) of gas at a predetermined pre-blowing pressure (P_(P)); an electrovalve (22) able to establish a communication between the inside of the preform (3), received in the cavity (15), and said source (20) of gas, at a predetermined pre-blowing flow rate; means (31) for controlling the opening of the electrovalve (22) at a predetermined moment called pre-blowing cue (t_(P)); a sensor able to measure the pressure (P) inside the preform (3); means for detecting the so-called development moment (t_(B)), when a pressure peak occurs in the preform (3); means for comparing the development moment (t_(B)) and the so-called development pressure (P_(B)), inside the preform (3) at this moment, with, respectively, a theoretical development moment and pressure; means for adjusting the pre-blowing flow rate according to this comparison.
 19. Machine (1) set forth in claim 18, comprising means for adjusting the pre-blowing pressure (P_(P)) according to the comparison between development moment and pressure with the theoretical development moment and pressure.
 20. Machine (1) set forth in claim 18, comprising means for adjusting the heating temperature according to the comparison between development moment (t_(B)) and pressure (P_(B)) with the theoretical development moment and pressure.
 21. Machine (1) according to claim 18, comprising means for adjusting the pre-blowing cue (t_(P)) according to the comparison between the development moment (t_(B)) and pressure (P_(B)) with the theoretical development moment and pressure.
 22. Machine (1) according to claim 18, comprising: a stretching rod (16) that can be displaced at a predetermined stretching speed (V_(E)); means for adjusting the stretching speed (V_(E)) according to the comparison between the development instant (t_(B)) and pressure (P_(B)) with the theoretical development moment and pressure.
 23. Computer programme product intended to be implemented on a machine (1) for manufacturing vessels according to claim 18, comprising instructions for: controlling the opening of the electrovalve (22); taking into account the development moment (t_(B)); taking into account the development pressure (P_(B)); taking into account the theoretical moment of development; taking into account the theoretical development pressure; comparing the development moment (t_(B)) with the theoretical moment of development; comparing the development pressure (P_(B)) with the theoretical development pressure; adjusting at least one of the following parameters: pre-blowing pressure (P_(P)), pre-blowing flow rate (D_(P)), pre-blowing cue (t_(P)), stretching speed (V_(E)), heating temperature (T) according to the result of these comparisons. 